Flat panel displays (FPDs) are increasingly replacing the conventional cathode ray tube (CRT) as the display type of choice. FPDs are electronic displays in which a flat screen is formed by a two-dimensional array of display elements (or “pixels”). They can be manufactured from a variety of different display technologies. One common display technology utilizes an array of light emitting diodes (LEDs) to form the FPD. An LED is a solid-state electronic device, more specifically a p-n junction or “diode”, which emits photons (i.e. light) when forward biased. The light emitting effect is referred to as injection electroluminescence, a light emitting phenomenon that occurs when minority charge carriers generated by an applied electric field recombine with charge carriers of the opposite type in the diode. The energy of the emitted photon, which determines the wavelength of the emitted light, varies with the band gap of the semiconductor material used (e.g., GaP, GaAs, GaN, etc.) to form the LED.
Typically, control of the LEDs in an FPD is performed using one of two approaches. According to the first approach, the LEDs are controlled by a row-column grid control pattern and associated row and column drivers/controllers. This approach is known as the “passive matrix” approach. The second approach, known as the “active matrix” approach, uses one or more control transistors at each pixel site to control pixel emission. Because each pixel is controlled by its own associated control transistor(s), active matrix LED FPDs consume less power than passive matrix FPDs, and are able to turn pixels on and off faster than passive matrix displays.
Another display technology of recent interest is based on the so-called Organic Light Emitting Diode (OLED). Operation of an OLED is similar to that of an inorganic semiconductor LED described above. When two organic materials, one with an excess of mobile electrons and the other with a deficiency, are placed in close contact, a junction region is formed. When a small forward bias is applied across the diode, electron-hole pairs are created, which upon recombination produce photons as described above. OLEDs are attractive for use in FPDs since they provide excellent display and viewing characteristics, can be manufactured on a flexible substrate (e.g. plastic), do not require high-temperature processing to dope them, and have fast element response times.
OLED FPDs are formed by etching an array of pixel elements into a substrate. In the array, portions of the active pixel elements, including thin film transistor (TFT) devices, storage capacitors and ITO patterns are formed on the substrate. The substrate is then coated with organic materials that form the light emitting portion (i.e. the diode) of the OLED. Further details concerning the manufacturing of OLED FPDs may be found in U.S. Pat. No. 5,688,551, which describes the first application of organic materials for OLED FPD manufacturing.
FIG. 1A shows a simplified diagram of a top view of a small six-column by four-row (6×4) OLED FPD 10. FPD 10 comprises an array of pixel elements 100, a row electrical driver 102, and a column electrical driver 104. During operation, if, for example, column electrical driver 104 activates column 3 and row electrical driver 102 activates row 2, then the pixel shown in black in FIG. 1A will be activated and light emission from this particular pixel element will result. A side view of the OLED FPD 10 in FIG. 1A is shown in FIG. 1B. There, the various layers of the FPD can be seen, including substrate 106, organic layer 108, and metal layer 109.
FIG. 1C shows a schematic diagram of a typical active pixel element 100 that is used in the OLED FPD in FIGS. 1A and 1B. Pixel element 100 is formed by two TFT devices 110 and 112, a storage capacitor 114, and an LED 116. TFT 110 acts like an analog electrical switch, which closes (i.e. turns ON) when the row selection signal 118 is active. Upon TFT 110 turning ON, the voltage present at column line 120 provides a charge source, which allows storage capacitor 114 to charge to a predetermined value. This charge is stored on storage capacitor 114, until a subsequent writing cycle corresponding to the display frame rate. As alluded to above in the discussion of non-organic LED pixel elements, this method of energizing a display pixel is referred to as “active”, due to the presence of TFT 110—an electronically active element. Active pixel elements are not unique to OLED FPDs. Indeed, for more information concerning active FPDs, reference may be made to the book “Color TFT Liquid Crystal Displays,” T. Yamasaki et al., edited by SEMI Standard FPD Technology Group, 1996.
FIG. 1D shows how the luminance of pixel element 100 is controlled. As shown, a voltage Vs present on storage capacitor 114 controls the transconductance (Gm) of TFT 112. A variation in Gm causes a variation in the current Id flowing into LED 116 and, consequently, the light emission luminance of LED 116. In essence, TFT 112 behaves like an electrically isolated voltage controlled current source in response to the voltage value Vs.
Turning now to the topic of defects in FPDs, it is well known that vast majority of defects in FPDs are found in the active plates of the FPDs. Because of this, during the manufacturing of FPDs, the active plates are typically tested prior to finally assembling the displays. By testing prior to final assembly, pixel defects can be detected early in the display manufacturing process, thereby resulting in a reduction in production costs.
Defects also commonly arise in the active plate of OLED displays. Accordingly, it would be desirable to test the active plates used in OLED displays prior to final assembly (e.g. prior to application of organic layer 108 and metal layer 109) as well. This desire is increased when it is recognized that organic layer 108 contributes substantially to the total display manufacturing costs. Besides material costs, a primary reason for the high cost is that atmospheric sealing methods must be employed to protect currently available organic emissive layers. Without proper protection from the atmosphere, the expected lifetime of organic layers can be substantially compromised. For more information on this topic, see the article “Microdisplays Based Upon Organic Light-Emitting Diodes,” W. E. Howard, et al., IBM J. RES. & DEV., vol. 45 no. 1, January 2001.
Unfortunately, testing the active plates prior to applying the organic and metal layers presents significant challenges since each pixel element output is in essence electrically floating, as shown schematically in FIGS. 2A and 2B. Specifically, FIG. 2A shows a top view of the OLED FPD plate prior to it being coated with the organic and metal layers 108 and 109 in FIG. 1A, and FIG. 2B shows that, because LED 116 is absent, each pixel element 100 is essentially a floating pixel element (fpe), i.e., an open electrical circuit.